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PubMed2003

Paradoxical vasodilation during lower body negative pressure in patients with vasodepressor carotid sinus syndrome.

OBJECTIVES: To elucidate the pathophysiological mechanism of the vasodepressor form (VD) of carotid sinus syndrome (CSS) by maneuvers designed to induce generalized sympathetic activation after baroreceptor unloading (lower body negative pressure, LBNP) or direct peripheral adrenoreceptor stimulation via local administration of norepinephrine (NA). DESIGN: Subjects were identified with VD of CSS through diagnostic testing. SETTING: Research laboratory. PARTICIPANTS: Eleven young controls (YC) (mean age +/- standard error of mean = 22.8 +/- 0.7), eight elderly controls (EC) (72.6 +/- 0.6), and eight elderly patients with VD (78.7 +/- 1.7). MEASUREMENTS: Forearm arterial blood flow (FABF) was measured in the left and right arms by venous occlusion plethysmography. Measurements were performed during baseline conditions, LBNP (-20 mmHg), and intra-arterial NA infusion in the left brachial artery at three progressively increasing rates (60, 120, and 240 pmol/min). RESULTS: During LBNP, FABF significantly decreased in YC (baseline 3.61 +/- 0.30 vs -20 mmHg 2.96 +/- 0.24 mL/100 g/min, P =.030) and EC (4.05 +/- 0.74 vs 3.69 +/- 0.65 mL/100 g/min, P =.033) but increased in elderly patients with VD (3.65 +/- 0.60 vs 4.54 +/- 0.80 mL/100 g/min, P =.020). During NA infusion, a significant forearm vasoconstriction occurred in YC (FABF left:right ratio 1.00 +/- 0.05 at baseline; 0.81 +/- 0.08 at 60 pmol/min, P =.034; 0.81 +/- 0.05 at 120 pmol/min, P <.001; 0.72 +/- 0.04 at 240 pmol/min, P <.001), whereas no significant FABF changes were observed in EC (1.04 +/- 0.06; 0.96 +/- 0.07, P =.655; 0.89 +/- 0.10, P =.401; 0.94 +/- 0.10, P =.590) or elderly patients with VD (1.04 +/- 0.06; 1.16 +/-0.10, P =.117; 1.04 +/- 0.08, P =.602; 1.11 +/- 0.10, P =.305). CONCLUSION: VD of CSS is associated with a paradoxical vasodilatation during LBNP and an impairment of peripheral alpha-adrenergic responsiveness, which may be age-related.

Adrenergic alpha-Agonists↗
PubMed1987

Effect of muscle tension on the cardiovascular responses to lower body negative pressure in man.

The purpose of this investigation was to evaluate the effect of moderate (non-hypertensive) levels of muscle tension on the cardiovascular responses to progressive lower body negative pressure (LBNP) in eight healthy male volunteers. Subjects were presented with progressive LBNP to -50 torr or the occurrence of vasovagal symptoms during three different levels of electromyographic activity in the lower limbs represented by the relaxed state, 5 and 10% maximal voluntary contraction. The same procedure was also performed at the same three levels of electromyographic activity in the arms with concomitant relaxation of the abdomen and lower extremities. In 75% of the subjects, pre-syncopal reactions were observed during the relaxed state while no pre-syncopal responses occurred during the elevated muscle tension levels. Both levels of muscle tension in the legs attenuated the LBNP-induced decrease in blood pressure (P less than 0.05). The effect of the 5% maximal voluntary contraction tension level appeared to be due to a compressive effect on the vascular tree, because similar levels of tension in the forearm had minimal effect on the blood pressure response to LBNP. In addition to a compressive effect, the 10% maximal voluntary contraction tension level appeared to induce a reflex stimulation of the heart as evidenced by an augmented heart rate response to LBNP and an increase in cardiac output. The muscle tension appeared to induce a mechanical compression of the vascular tree which was accompanied by somatopressor reflex responses, resulting in a maintenance of blood pressure that was primarily mediated by a maintained cardiac output.

Adult↗
PubMed1993

Use of lower body negative pressure to counter symptoms of orthostatic intolerance in patients, bed rest subjects, and astronauts.

This report briefly discusses some aspects of autonomic cardiovascular dysfunction as related to changes in orthostatic function in patients, bed rest subjects, and astronauts. This relationship is described in normal individuals to provide the basis for discussion of parameters that may be altered in patients, bed rest subjects, and astronauts. The relationships between disease states, age, periods of weightlessness during space flight, and autonomic dysfunction, and their contribution to changes in orthostatic tolerance are presented. The physiologic effects of lower body negative pressure are illustrated by presenting data obtained in bed rest subjects and in astronauts. Finally, the usefulness of lower body negative pressure to counter symptoms of orthostatic intolerance in patients, bed rest subjects, and astronauts is discussed.

Autonomic Nervous System Diseases↗
PubMed1989

Regional vascular responses to prolonged lower body negative pressure in normal subjects.

The purpose of this study was to determine the effects of sustained unloading of baroreceptors in humans. The regional hemodynamic responses to lower body negative pressure (LBNP) were determined in 20 normal subjects. LBNP at -10 mmHg for 1 h decreased central venous pressure (CVP) without affecting blood pressure or heart rate, suggesting that only cardiopulmonary baroreceptors were unloaded. Forearm blood flow (FBF) and splanchnic blood flow (SBF) both decreased. Renal blood flow (RBF) did not change, but glomerular filtration rate (GFR) increased. Plasma renin activity rose slightly, whereas plasma norepinephrine levels did not change. Peak LBNP (either -20 or -40 mmHg for 1 h) caused a further decline in CVP and narrowed pulse pressure, thus unloading both arterial and cardiopulmonary baroreceptors. FBF returned to base-line values and SBF decreased further. RBF fell and the GFR remained increased. Plasma renin activity increased further, and plasma norepinephrine level rose. Thus the forearm and splanchnic circulations are sensitive to sustained unloading of cardiopulmonary baroreceptors; renal vasoconstriction occurs with additional unloading of arterial baroreceptors. Renin-angiotensin system activation during LBNP may be pertinent to the preservation of glomerular filtration.

Adult↗
PubMed1986

Forearm skin and muscle vasoconstriction during lower body negative pressure.

In view of conflicting reports of skeletal muscle and skin blood flow participation in baroreceptor-mediated reflexes, we studied the effects of graded lower body negative pressure (LBNP) on cutaneous and muscular components of forearm blood flow (FBF) in seven male subjects at 28 degrees C. FBF was measured by venous occlusion plethysmography and cutaneous flow by laser-Doppler velocimetry, the difference being the muscular flow. Mean FBF decreased by 39 and 56% from control at LBNP of 20 and 50 Torr, respectively. Skin flow decreased linearly with graded LBNP contributing 32% of the decrease of total blood flow at 20 Torr and then 50% of total decrease of blood flow at 50 Torr. Conversely, the decrease in muscle flow represented 68% of the total decrease at LBNP of 20 Torr and then 50% of the total decrease at LBNP of 50 Torr. We concluded that both skin and muscle circulations participate in sustained peripheral vasoconstriction during LBNP, with muscle flow achieving near maximum vasoconstriction by 20 Torr and skin showing a graded vasoconstriction to decreases in LBNP.

Adult↗
PubMed2001

[Changes of cardiac function during 21 d head-down tilt bed rest and the effect of lower body negative pressure in the last week].

OBJECTIVE: To investigate the change of cardiac function during 21 d head-down tilt (HDT) bed rest and the effect of lower body negative pressure (LBNP) in the last week in human. METHOD: 12 healthy male served as subjects which were randomly divided into control and LBNP groups, with 6 in each group. All of them were exposed to -6 degrees HDT for 21 d. The LBNP group received -4.0 kPa LBNP training 1 h/d in the last week of the test. The cardiac pump function and cardiac systole function were measured before, during and after HDT. RESULT: Cardiac output, cardiac index and stroke volume decreased significantly, whereas total peripheral resistance, pre-ejection period (PEP), isovolumetric contraction time/left ventricular ejection time (LVET) and PEP/LVET increased significantly during HDT in both control and LBNP groups, compared to pre-HDT values, and they returned to normal level on day 2 post HDT. PEP and PEP/LVET in LBNP group were significantly higher than those in control group on day 21 of HDT and day 2 post HDT. CONCLUSION: The reduction of cardiac pump function and cardiac systole function induced by 21 d HDT could not be prevented by LBNP training in the last week of 21 d HDT.

Adolescent↗
PubMed2004

Stroke volume and sympathetic responses to lower-body negative pressure reveal new insight into circulatory shock in humans.

We measured various hemodynamic responses and muscle sympathetic nerve activity (MSNA) in human subjects during a graded lower-body negative pressure (LBNP) protocol to test the hypotheses that: (1) reduced stroke volume (SV) is linearly related to increased MSNA; and (2) the onset of symptoms of impending cardiovascular collapse is associated with hypoadrenergic responses to central hypovolemia. We measured heart rates, arterial blood pressures, sympathetic neural activity (MSNA; peroneal nerve microneurography), and relative changes (% Delta) in SV (thoracic electrical bioimpedance) in 13 men during exposure to graded levels of LBNP. After a 12-min baseline data collection period, LBNP was initiated at -15 mm Hg for 12 min followed by continuous stepwise increments to -30, -45, and -60 mm Hg for 12 min each. Eight subjects completed the LBNP protocol (finishers), while the protocol was terminated prematurely during -60 mm Hg in five subjects due to onset of symptoms of cardiovascular collapse (nonfinishers). Of these subjects, we were able to record MSNA successfully throughout the LBNP protocol in four finishers and two nonfinishers. The relationship between average change in stroke volume and average change in MSNA was linear (% DeltaMSNA=464-3.6 [% DeltaSV], r2=0.98). On average, MSNA was greater in the nonfinishers at each level of LBNP compared to finishers, but peripheral resistance was lower. Our results support the hypothesis that MSNA activation is inversely related and linear to stroke volume reductions during central hypovolemia. Sympathetic withdrawal rather than hypoadrenergic function may represent a fundamental mechanism for the development of circulatory shock.

Adult↗
PubMed1998

Effects of exercise training mode on the cardiovascular responses to lower body negative pressure in males.

BACKGROUND: Endurance-trained males may be more prone to orthostatic hypotension than untrained subjects and this is reflected in differences in their cardiovascular responses to lower body negative pressure (LBNP). It is uncertain if the type of endurance training used affects these responses. HYPOTHESIS: Endurance-trained runners will differ from endurance-trained swimmers in their cardiovascular responses to LBNP. METHODS: Male intercollegiate cross country runners (XC, n = 9), sprinters (SP, n = 7) and swimmers (SW, n = 12) underwent exposures to -10, -20 and -40 mm Hg LBNP. Forearm blood flow, heart rate (HR) and blood pressure were measured throughout. Maximal oxygen consumption (VO2max) was determined separately via combined arm-and-leg cycle ergometry. RESULTS: The XC were more fit than the SW and SP (VO2max = 64.5 +/- 3.5 vs. 51.7 +/- 1.9, 49.5 +/- 2.4 ml.kg-1.min-1; x +/- SEM; p < 0.05). Resting mean arterial pressure (MAP, 93 +/- 3 mm Hg) and forearm vascular resistance (FVR; 26.2 +/- 6.0 units) did not differ significantly between groups although HR was 17 +/- 3 b.min-1 higher (p < 0.05) in SW vs. SP and XC. Neither the pulse pressure, MAP, HR, nor FVR responses to the LBNP exposures differed significantly between the 3 groups. Compared to rest, pulse pressure was reduced (p < 0.05) 14% at -40 mm Hg. Similarly, HR was increased (p < 0.05) 10% at -40 mm Hg. FVR increased (p < 0.05) with each increase in LBNP becoming 91% greater at -40 mm Hg vs. rest. CONCLUSIONS: These data suggest that the cardiovascular responses to LBNP up to -40 mm Hg do not differ in chronically exercising males training with different modalities.

Adult↗
PubMed1998

Pulmonary capillary blood volume during lower body negative pressure: effect of gender.

BACKGROUND: The purpose of this study was to examine the changes in pulmonary capillary blood volume (VC) in men and women during lower body negative pressure (LBNP). Additionally, the components of lung diffusion capacity were measured and evaluated for the effect of gender and LBNP. METHODS: There were 6 men and 6 women who underwent a staged LBNP protocol to -40 mm Hg. The diffusion of the lung for carbon monoxide (DLCO) was determined at two levels of inspired oxygen (21% and 60%). DLCO was separated into the diffusion capacity of the membrane (DLM) and the diffusion capacity of blood. RESULTS: During supine rest, DLCO values for men were larger than for women and were 39.8 +/- 3.8 and 32.2 +/- 2.2 ml.min-1.mm-1 Hg, respectfully. DLCO decreased equally with each stage of LBNP for men and women. VC during supine rest was greater for men (131 +/- 8 ml) than for women (92.7 +/- 7 ml). VC also declined with each stage of LBNP, and the decline was similar for men and women. DLM did not change with LBNP. CONCLUSION: The decrease in DLCO with LBNP is due to the reductions in thoracic blood volume in both men and women. These thoracic blood volume changes do not explain the previously reported reduced tolerances to LBNP for women because the reductions in thoracic blood volume were similar for men and women.

Adult↗
PubMed2001

[Changes of cerebral blood flow during 21 d head-down tilt bed rest and the effect of lower body negative pressure during the last week].

Objective. To investigate the changes of cerebral blood flow (CBF) during 21 d head-down tilt (HDT) bed rest and the effect of lower body negative pressure (LBNP) in the last week. Method. Twelve healthy male subjects were randomly divided into control and LBNP groups, with 6 in each group. All of them were exposed to -6 degrees HDT for 21 d. The LBNP group received -4.0 kPa LBNP training 1 h/d in the last week of HDT while the control group did not. CBF and cerebral vascular resistance were measured by rheoencephalogram in pre-HDT, day 3, 10 and 21 of HDT. Result. In control group, left cerebral I quadrant area and inrush velocity decreased significantly during HDT, and left cerebral delta Gy wave crest height decreased significantly, whereas left cerebral resistance index increased significantly on day 3 and 21 of HDT as compared to those of pre-HDT. In LBNP group, left cerebral I quadrant area decreased significantly, and left cerebral delta Gy wave crest height and inrush velocity tended to decrease on day 3 and 21 of HDT, whereas left cerebral resistance index increased significantly during HDT as compared to those of pre-HDT. There were no significant differences between above indexes in control group and LBNP group. Conclusion. It is suggested that 21 d HDT may increase cerebral vascular resistance and decrease CBF, which can not be prevented by LBNP training in the last week of 21 d HDT.

Adult↗
PubMed2002

Sympathetic nervous and hemodynamic responses to lower body negative pressure in hyperbaria in men.

The present study was designed to test the hypothesis that sympathetic nerve activity is attenuated in a hyperbaric environment. Response of muscle sympathetic nerve activity (MSNA) to central circulatory hypovolemic stress, lower body negative pressure (LBNP), was measured in nine men at normal and at 3 atm pressures. The stress consisted of 4 min each of control and LBNP at -20 and -40 mmHg. In addition to MSNA, heart rate, stroke volume (SV), forearm blood flow (FBF), and volume of the lower leg were recorded. A reduction of baseline HR occurred with increased forearm vascular resistance at 3 atm abs. The baseline MSNA decreased during hyperbaria. MSNA increased progressively with increasing LBNP in both atmospheric pressures, and the change from the baseline (DeltaMSNA) was similar in both conditions. Changes in SV, FBF, and volume of the lower legs in response to LBNP were not statistically different during exposure to 2 atm pressures. The present study suggests that hyperbaria attenuates sympathetic nerve activity; however, its responsiveness to hypovolemic stress was not affected by hyperbaric exposure.

Adult↗
PubMed2000

Supine lower body negative pressure exercise simulates metabolic and kinetic features of upright exercise.

Exercise within an artificial gravity environment may help prevent microgravity-induced deconditioning. We hypothesized that supine lower body negative pressure (LBNP) exercise simulates physiological and biomechanical features of upright exercise. Walking (4.5 +/- 0.3 km/h) and running (8.0 +/- 1.0 km/h) while supine within a LBNP exerciser were compared with walking and running while upright. Eight healthy subjects exercised for 5 min at each of the four posture/gait conditions. LBNP of 52 +/- 4 mmHg generated one body weight of supine ground reaction force (GRF). Gait parameters and GRFs were measured during the third minute of exercise, and heart rate and oxygen consumption were measured during the fifth minute. Oxygen consumption during supine LBNP treadmill exercise [walking: 14.6 +/- 0.9; running: 32.2 +/- 1.6 (SE) ml. min(-1). kg(-1)] was similar to that during upright treadmill exercise (walking: 15.1 +/- 0.9; running: 34.0 +/- 1.9 ml. min(-1). kg(-1)). Heart rate for supine LBNP exercise (grand mean: 133 +/- 11 beats/min) was also similar to that for upright exercise (136 +/- 11 beats/min). Footward forces integrated over each stride (330.5 +/- 34.4 vs. 319. 1 +/- 29.6 N. s) and rate of force generation (26,483 +/- 4,310 vs. 25,634 +/- 4,434 N/s) were similar for upright and LBNP exercise, respectively. Our collective results indicate that supine exercise within LBNP can simulate the physiological stress and GRFs that are generated during upright gait.

Adult↗
PubMed2001

[Effect of lower body negative pressure and rotating-table simulated push-pull effect in flight on cardiovascular function].

Objective. To explore the effect of push-pull effect (PPE) simulated by lower body negative pressure (LBNP) rotating-table, and observe the physiological responses to push-pull maneuver. Method. A special LBNP rotating-table was used to simulate the push-pull maneuver. 8 healthy adults participated randomly in two experiments. One was simulated PPE test, which include a series of head-up stand (HUT, +1 Gz) for 1 min, then in head-down stand (HDT, -1 Gz) for 30 s and again in HUT combining LBNP (-50 mmHg) for 10 min. The other one was control test, which only consists of HUT combining LBNP (-50 mmHg) for 10 min. Changes of heart rate (HR), blood pressure (BP), basic impedance (Z0) , stroke output (SO) , cardiac output (CO) and total peripheral resistance (TPR) were monitored by electrical impedance instrument during the experiment. Result. During simulated PPE experiment, 3 subjects presented presyncopal symptoms, with average standing time of 8.99 +/- 1.47 min, while during control test, all the subjects completed HUT combining LBNP for 10 min. In simulated PPE experiment, as compared with HUT (control), HR, Z0 during HDT were significantly lowered, while SV and CO were increased significantly. During HUT + LBNP, HR, Z0 and TPR were significantly higher, while SV and CO were significantly lower than that of control and HDT. SBP was increased significantly than control value when "HUT + LBNP" started, but during the whole process of "HUT + LBNP ", it became significantly lowered. In control experiment, the above mentioned indexes showed the same trend of as change compared with the control, however, the percentage of the change was lower than simulated PPE test, the change percentage in HR was not including. Conclusion. After headstand, head-up stand combining LBNP caused cardiovascular function descends, the degree was larger than simple head-up stand combining LBNP. LBNP rotating-table can be used to simulated push-pull effect.

Adult↗
PubMed1995

Effects of 28-day head-down tilt with and without countermeasures on lower body negative pressure responses.

This study was performed to determine the effects of 28 d of head-down tilt (HDT) (simulated weightlessness) on cardiovascular responses to orthostatic stress induced by lower body negative pressure (LBNP) (before, during, and after HDT) and +60 degrees head-up tilt (before and after HDT) in 12 subjects. Half of them underwent countermeasures (CM) of regular muscular exercise (isometric and isokinetic training) and LBNP sessions (-30 mm Hg) as generally performed during spaceflight; the other six were a control group (C). The countermeasure effect on the orthostatic responses to LBNP and tilt test was assessed by studying the changes after HDT in the two groups. Essentially, blood pressure was better maintained in group CM in the tilt test after HDT (MBP at the end of the tilt vs. baseline value: +16% (CM); -19% (C)). LBNP and muscular exercise may have contributed to this improvement. One of the probable contributing factors is the relative conservation of plasma volume, at the end of HDT, in group CM (-2.2%), compared to group C (-11.2%). Transcranial Doppler (TCD) recordings of middle cerebral artery (MCA) velocities permitted indirect evaluation of cerebral blood flow changes during the orthostatic tests. MCA velocities decreased significantly although slightly (-7 to -12%) during LBNP sessions without changes along the HDT showing that the cerebral circulation was well preserved in each group. On the other hand, subjects undergoing presyncopal symptoms presented a drop in MCA velocities, suggesting a decrease in cerebral blood flow.

Adult↗
PubMed1987

Hysteresis in response to descending and ascending lower-body negative pressure.

We have investigated the pattern of fluid redistribution and cardiovascular responses during graduated orthostatic stress. Twelve men, age 30-39 yr, underwent a 25-min lower-body negative pressure (LBNP) test protocol that involved sequential stages of LBNP at -8 mmHg (1 min), -16 mmHg (1 min), -30 mmHg (3 min), -40 mmHg (5 min), -50 mmHg (5 min), -40 mmHg (5 min), -30 mmHg (3 min), -16 mmHg (1 min), and -8 mmHg (1 min). Data were recorded at the end of each stage. For many measured variables values during the descending phase of LBNP (-8 to -40 mmHg) were significantly different from values during the ascending phase of (-40 to -8 mmHg). These differences appear to be due to a component of fluid translocation that occurs during LBNP and cannot be reversed within the duration of the procedure. We hypothesize that this slowly reversed component is sequestration of fluid in the interstitial and lymphatic compartments. In contrast, venous pooling is a rapidly reversible component of fluid movement during LBNP. A scheme describing fluid and cardiovascular responses to LBNP based on these data and the data of other investigators is presented.

Adult↗
PubMed1994

Changes in body core temperatures and heat balance after an abrupt release of lower body negative pressure in humans.

Changes in body core temperature (T(cor)) and heat balance after an abrupt release of lower body negative pressure (LBNP) were investigated in 5 volunteers under the following conditions: (1) an ambient temperature (Ta) of 20 degrees C or (2) 35 degrees C, and (3) Ta of 25 degrees C with a leg skin temperature of 30 degrees C or (4) 35 degrees C. The leg skin temperature was controlled with water perfusion devices wound around the legs. Rectal (T(re)), tympanic (T(ty)) and esophageal (T(es)) temperatures, skin temperatures (7 sites) and oxygen consumption were measured. The intensity of LBNP was adjusted so that the amount of blood pooled in the legs was the same under all conditions. When a thermal balance was attained during LBNP, application of LBNP was suddenly halted. The skin temperatures increased significantly after the release of LBNP under all conditions, while oxygen consumption hardly changed. The release of LBNP caused significant falls in T(cor)s under conditions (1) and (3), but lowered T(cor)s very slightly under conditions (2) and (4). The changes in T(es) were always more rapid and greater than those of T(ty) and T(re). The falls in T(ty) and T(re) appeared to be explained by changes in heat balance, whereas the sharp drop of T(es) could not be explained especially during the first 8 min after the release of LBNP. The results suggest that a fall in T(cor) after a release of LBNP is attributed to an increase in heat loss due to reflexive skin vasodilation and is dependent on the temperature of venous blood returning from the lower body.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed2002

Age-related influences of leg vein filling and emptying on blood volume redistribution and sympathetic reflex during lower body negative pressure in humans.

To test the hypothesis that leg vein filling and emptying functions could be impaired with advancing age, which would produce less blood volume redistribution toward the lower body and smaller sympathetic reflex response during mild gravitational stress, 9 young and 10 elderly healthy males were exposed to a lower body negative pressure (LBNP) of 15 mmHg. Venous occlusion plethysmography was used to determine the functions of the leg veins. We found that the baseline venous distensibility index (VDI) was lower (0.057 +/- 0.004 vs. 0.048 +/- 0.003 ml x 100 ml(-1) x mmHg(-1), young vs. elderly; p < 0.05), and half-emptying time (T(1/2)) was shorter (1.6 +/- 0.1 vs. 1.3 +/- 0.1 s, young vs. elderly; p < 0.05) in the elderly. At 15 mmHg-LBNP, VDI was decreased and T(1/2) was shortened significantly in the young group, but only slightly in the elderly group. Neither blood pressure nor heart rate changed significantly in either group. The reduction in peripheral venous pressure, which was recorded from the left antecubital vein at the cubital fossa, was less in the elderly, indicating a smaller decrease in central blood volume during LBNP; however, the enhancement of muscle sympathetic nerve activity was nearly the same as that in the young. We conclude that leg vein filling and emptying functions are impaired in elderly people, producing less blood pooling in the legs and smaller reduction in peripheral venous pressure during LBNP; the maintained sympathetic reflex response might be attributable to the well-preserved baroreflex function control of sympathetic outflow to the muscle in the elderly.

Adult↗
PubMed1979

Comparison of various vasodilator drugs using lower body negative pressure.

The present investigations were designed to enable a differentiation in vivo between various types of vasodilator drugs by studying their influence on mean arterial blood pressure (MAP) in anaesthetized rats, simultaneously subjected to lower body negative pressure (LBNP). In anaesthetized rats the application of various degrees of LBNP caused a proportional decrease in MAP, cardiac output and central venous pressure, whereas heart rate remained unchanged. Reflex tachycardia was not observed as a result of general anaesthesia. The autonomic nervous system does not play a part in the physiological events induced by LBNP, since various pretreatments (vagotomy, adrenalectomy, pharmacological sympathectomy) did not change the response to LBNP. It is concluded that the reduction in MAP due to LBNP is a purely mechanical, haemodynamic phenomenon. Upon combined application of drugs and LBNP, vasodilator drugs which mainly dilate veins (sodium nitrite, isosorbide, nitroglycerine) were more active with respect to their hypotensive action at a LBNP of 5 mm than at 3 mm Hg. However, vasodilator drugs with a predominantly arterial site of action (hydralazine, papaverine, phentolamine, diazoxide) were more potent at a LBNP of 3 mm than at 5 mm Hg. This different behavior may be used to discriminate in vivo between various types of vasodilator drugs. The method is accurate, reproduci0le and relatively easy to perform.

Animals↗